1. Field of the Invention
This invention relates to a charged particle beam apparatus having a column inclined with respect to a sample surface.
2. Description of the Related Art
To observe or process a very fine pattern formed on a semiconductor wafer, charged particle beam apparatuses such as a scanning electron microscope (SEM) and a focused ion beam apparatus (FIB) have been used to this date.
JP-A-2002-503870 discloses an improved focused ion beam apparatus having a construction in which axes of two columns of FIB and SEM are inclined at 45° to a Z axis, their axes have a perpendicular relation and no offset exists between the axes.
JP-A-2002-503870 does not mention the case where offset exists between the axes of FIB and SEM. This reference does not mention the problem of deviation of visual fields of SEM and FIB resulting from the difference of a sample height and the problem of deviation of the visual fields after focus adjustment of a beam. Furthermore, the reference does not at all mention the problem of a read error of a position resulting from the difference of the height of an alignment point that is unavoidable in gradient columns.
In defect inspection of semiconductor wafers, it is necessary to analyze a defect position detected by an inspection apparatus by another apparatus. Because coordinates systems are different between these apparatuses (center of wafer coordinates system, rotation, distortion of X and Y, etc resulting from the difference of measurement system of stage), correction must be made by conducting wafer alignment between the apparatuses and then positioning. To move the stage to the defect position detected by the wafer inspection apparatus and to find out the defect from the screen, the defect must fall within at least the visual field of the image at the movement destination. To identify the defect, magnification capable of recognizing at least the defect is necessary. Therefore, the magnification necessary for identifying the defect is estimated. When the size of the defect becomes 0.1 μm, the size of the defect is 1 mm at magnification ×10 k and is displayed at 14 μm at an angle of view of 140 mm. The defect can be specified relatively easily when the defect can be enlarged up to 2 mm. This magnification is ×20 k and is display of 7 μm. Assuming that integration of devices further proceeds and the existing 90 nm node becomes 65 nm node in 2005 or 2006, the defect of the devices becomes smaller with the node. Therefore, to enlarge the defect of 65 nm to 2 mm and to search the defect, magnification ×30 k is approximately necessary and display of less than 5 μm is necessary. In other words, positioning accuracy of ±3 μm or below is necessary.
In ordinary wafer alignment executed by a column vertical to the wafer, the focus changes with the change of the wafer height but the position does not change. Therefore, positioning can be made with high accuracy by wafer alignment without taking the wafer height into consideration. In the case of the column apparatus inclined with respect to the wafer, however, the position changes depending on the height of the point at which wafer alignment is made. It is therefore necessary to always keep the wafer alignment point at a predetermined reference height. Nonetheless, the difference of the height exists inside the wafer surface because the wafer generally has warp.
When the beam of each of FIB and SEM is inclined to the axis Z perpendicular to the X, Y plane of the stage on which the wafer is put and when a point P1 at the height at which the point of intersection of the two beams exists as shown in FIG. 1, the point P1 can be observed without moving the stage. When the height of the observation point is not Z0, the observation visual fields of these two beams are different and the stage movement L is necessary for establishing focus of FIB on the same observation point when a point P2 at a certain height h is observed through SEM.
When SEM and FIB are used for wafer inspection as described above, the inspection apparatus must be move with accuracy of about 3 μm to a position designated by the defect inspection apparatus. In the case of the column apparatus perpendicular to the stage, the observation position (X, Y) doe not change even when the height of the observation point changes. Therefore, positioning can be made with accuracy of about 3 μm by executing wafer alignment. In the gradient column, however, the observation position (X, Y) changes when the height of the observation point changes because observation is oblique observation. When the processing object is the wafer, the wafer undergoes warp and distortion with the progress of the semiconductor process such as washing, exposure, development, vacuum deposition, etching, heat treatment and so forth. In consequence, warp of a φ300 mm wafer is as great as 200 μm. This warp results as such in the deviation of the visual field of the image in the case of the 45° gradient column. Consequently, alignment accuracy drops and eventually, positioning accuracy gets deteriorated. In this case, positioning accuracy drops unless the offset amount is correctly grasped.